Photochromic and fluorescent LC gels based on a bent-shaped azobenzene-containing gelator

Abstract

Photochromic LC-gels based on a low-molar-mass azobenzene-containing bent-shaped gelator and nematic liquid crystals were prepared. LC-gels are capable of reversible melting under E–Z isomerization of azobenzene chromophores induced by UV-irradiation. It is shown that light and heat actions allow one to manipulate the phase behaviour, fluidity and optical properties of the prepared LC-gels. The observed phenomenon was applied for the creation of nematic and cholesteric mixtures with phototunable degrees of linearly or circularly polarized fluorescence. The elaborated systems can be considered as promising soft-matter materials for optics and photonics applications.

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Among the different types of smart materials, gels are the one of the most promising stimuli-responsive systems which have gained significant interest due to their large variety of unique properties. In particular, there are the possibilities of fast and reversible switching of the mechanical properties under different fields and actions, such as light, pH changes, mechanical force, etc. Of special interest are light-responsive gels^1–13 because light is one of the most useful and simple tools for fast, distant and localized modification of the mechanical and optical properties of materials, including gels.

Another promising type of responsive system includes liquid crystalline (LC) gels capable of forming anisotropic films or layers with photo-, thermo- and electro-switchable optical properties, allowing the creation of novel types of materials with controllable supramolecular structure.^11–22 In the paper by Kato et al.,¹³ the photoinduced gel–sol transition in photochromic LC-gels with hydrogen-bonded azobenzene-containing gelators was described. The authors have demonstrated the possibilities of photo-optical patterning and photorecording, but they did not consider in their work the effects of photofluidization under UV-irradiation.

Recently, we have used a photochromic azobenzene-containing LC acrylic polymer for the gelation of a low-molar-mass nematic mixture.²² It was shown that the dissolution of the LC polymer in an amount of only 2.5 wt% in the nematic mixture MLC6816 (Merck Ltd) at 120 °C (which corresponded to the isotropic state) followed by cooling down, results in the formation of a solid-like photochromic LC gel. Gelation is associated with a phase separation and the formation of microsized LC polymer domains which form a physical “network” containing an encapsulated nematic host. It was found that the UV-irradiation leading to the E–Z isomerization processes is accompanied by a disruption of the H-aggregates formed by the azobenzene moieties and by partial dissolution of the polymer. Nevertheless, in this work we did not find any evidence of isothermal photoinduced melting of the gel.

In the current short preliminary communication we are presenting the first results of the investigation of a LC gel with a photooptically induced gel–sol transition. We have examined a lot of liquid crystal–gelator pairs in order to find the substances those allow the formation of LC gels capable of isothermal photoinduced melting. As a result of this search, we have found that a new bent-shaped compound, 10BVIABr, could be successfully used as a gelator for the LC mixture MLC6816 (Scheme 1a). The synthesis and detailed characterization of the novel photochromic azobenzene-containing bent-shaped compound, 10BVIABr, are presented in the ESI (Fig. S1–S5†).

Scheme 1 (a) The structure of the nematic LC mixture, MLC6816, and of the photochromic bent-shaped gelator, 10BVIABr. (b) E–Z isomerization of the bent-shaped molecule of 10BVIABr.

Heating of the nematic MLC6816 mixture doped with 1.0–2.6 wt% of the bent-shaped compound 10BVIABr up to 120 °C results in the complete mutual dissolution of the components and the formation of a transparent isotropic liquid. Subsequent cooling of the liquid to room temperature induces an appearance of strong turbidity due to the isotropic phase–nematic phase transition. This process is accompanied by the slow formation of the gel which is completed after ca. 10 hours (Fig. 1a). The gel formation is most probably associated with the formation of a physical “solid network” composed of 10BVIABr crystallites (Scheme 2). As seen from Fig. 1c, the DSC curves of the nematic gels contain two peaks, one of them corresponds to the melting transition of the gel (at ca. 70 °C), whereas the peak at 81 °C relates to the isotropization of the sample. These transitions are also confirmed using polarized optical microscopy (POM) observations.

Fig. 1 Photos of the LC mixture containing 1.0 wt% of the gelator, 10BVIABr, before (a) and after (b) UV-irradiation (380 nm, ∼8 mW cm⁻², 10 min). (c) DSC curve of the nematic gel containing 2.6 wt% of the gelator (first heating scan obtained with a rate of 10° per min).

Scheme 2 A schematic representation of the LC-gel structure and its melting under UV-irradiation.

It is noteworthy that UV-irradiation of the LC gel for only 10 min with moderate intensity light results in the melting of the gel or a photoinduced gel–sol transition and the appearance of an intense orange colour instead of a pale yellow one (Fig. 1b).

Both effects are associated with the process of E–Z isomerization of the azobenzene photochromic fragment of 10BVIABr (Scheme 1b) that leads to the photoinduced melting of the 10BVIABr crystals and dissolving of the formed Z-isomer in the LC mixture (Scheme 2).

The origin of these effects is related to the fact that the Z-isomer of the 10BVIABr molecules possesses less anisometry, compared to the E-form (Scheme 1b). At room temperature the mixture exposed to UV-irradiation regains the original yellow colour in about 3 days, but the reverse gelation does not occur. Only the precipitation of the yellow crystals of 10BVIABr takes place. However, reheating the mixture to 120 °C followed by cooling down and keeping it at room temperature again, for approximately 12 hours, leads to the gelation process.

Polarized optical microscopy (POM) confirmed that the gel formation occurs due to the crystallization of the azobenzene gelator (Fig. 2a and b), whereas UV-irradiation leads to the complete melting of the crystals and their dissolution (Fig. 2c and Scheme 2).

Fig. 2 (a) Photomicrographs taken using polarized optical microscopy in crossed polarizers for the mixture containing 1 wt% of the gelator just after cooling from 120 °C to room temperature. (b) The same cell after annealing at room temperature for two days. (c) After UV-irradiation (380 nm, ∼8 mW cm⁻², 10 min). The cell thickness is 5 μm; the glass substrates are coated with a polyvinyl alcohol layer uniaxially rubbed in one direction. At the right of the Figure, the same samples rotated at 45 °C with respect to the left ones are shown. Scale bar is 100 μm.

We have studied the spectral properties of the photochromic photoswitchable LC-gel. Fig. 3 shows the absorption spectra of the cell filled with LC gel during UV irradiation (Fig. 3a) and subsequent exposure to visible light (Fig. 3b). As is seen from Fig. 3a, the gel has a low light transmission in the whole spectral range, which is caused by light scattering due to the presence of two phases, – the LC nematic and crystalline. A small “shoulder” on the spectrum at wavelengths around 360 nm is due to the π–π* electronic transition of the azobenzene chromophores in the E-form. UV-irradiation results in the lowering of the absorption in the whole spectral range caused by the E–Z isomerization, melting of the gel and dopant dissolution (Fig. 3a). In the photostationary state, a peak with a maximum around 450 nm is clearly seen, which corresponds to the n–π* electronic transition of the azobenzene chromophore. Subsequent irradiation with visible light leads to a decrease in the intensity of the n–π* electronic transition and the strong increase in the absorbance corresponding to the π–π* electronic transition of the azobenzene chromophores in the E-form. Simultaneously, the increase in absorbance in the whole region of the spectrum due to the crystallization process of the E-isomer occurring during the reverse Z–E isomerization and gelation are observed.

Fig. 3 Changes in the absorbance spectra of the gel containing 2.6 wt% of the gelator during (a) UV-irradiation (380 nm, ∼8 mW cm⁻²) and (b) subsequent visible light irradiation (436 nm, ∼1.0 mW cm⁻²). Cell thickness is 30 μm.

The action of UV-light results not only in changes in fluidity and optical properties, but also leads to a significant improvement of the LC-alignment in cells with the uniaxially rubbed polymer-coated glass substrates (Fig. 2b and c). Just after gelation, the uniaxial alignment is disrupted by a dense physical network formation consisting of the crystallites of 10BVIABr (Fig. 2b). The photoinduced melting of the crystals induces good alignment of the LC molecules in the cell (Fig. 2c).

The observed phenomenon of the photoinduced gel–sol transition was applied for the photomodulation of the fluorescent properties of the LC-mixtures doped with fluorescent dyes. The nematic mixture containing the gelator was doped with 0.2 wt% of the dye DCM2, possessing fluorescence in the red spectral region.

The nematic mixture in the gelled state, sandwiched in the uniaxially aligned cell, has a relatively low degree of fluorescence polarization (Fig. 4a). The values of emission anisotropy, R, were calculated using eqn (1):

R = (I_∥ − I_⊥)/(I_∥ + I_⊥) (1)

where I_∥ and I_⊥ are the intensities of light emission polarized parallel and perpendicular to the alignment direction, respectively. The R value for the LC-gel is 0.16; such a low value is explained by the above-mentioned formation of the dense network of gelator crystals, preventing a good alignment of the LC and dye molecules. Subsequent UV-irradiation and the photoinduced melting of the gel results in a strong increase in the degree of polarization up to R = 0.36 (Fig. 4a), due to the strong improvement of the molecular alignment in the LC-cell (see also the POM images in Fig. 2b and c).

Fig. 4 (a) Polarized fluorescence spectra of the aligned cell (5 μm), filled with the mixture of MLC6816 + 10BVIABr 1.0% + DCM2 0.2%, before and after the photoinduced gel–sol transition (10 min of UV-irradiation, 380 nm). The excitation wavelength is 532 nm. Anisotropy of the fluorescence polarization at 607 nm before irradiation, R = 0.16, and after irradiation, R = 0.36. (b) Changes in the dissymmetry factor after the photoinduced gel–sol transition during UV-irradiation (380 nm, 20 min) of a cholesteric mixture (MLC6816 48.8% + CholPel 25.9% + CholVal 23.6% + 10BVIABr 1.5% + DCM2 0.2%).

The same idea was applied for the preparation of the cholesteric mixture with phototunable circularly-polarized fluorescence. For this purpose, the nematic gel was doped with the mixture containing the chiral dopants cholesteryl pelargonate (CholPel, 25.9%) and cholesteryl valerate (CholVal, 23.6%). Combination of two chiral substances was used in order to avoid any phase separation in the mixture and improve miscibility of the components. As for the nematic gel, UV-irradiation results in the gel–sol transition and melting of the cholesteric gel (Fig. S6†). The concentration of the chiral substances was selected to obtain selective light reflection in the red spectral region (Fig. S7†) coinciding with DCM2 emission. The obtained mixture forms a left-handed helical cholesteric structure. Overlapping of the selective light reflection peak (or photonic band gap) of the cholesteric phase and fluorescence peak allows one to obtain strong circular polarization of the emitted light.^23–25 As seen from the spectra of circularly-polarized fluorescence (Fig. S8†), the left-handed component of the emission has a pronounced gap in intensity which coincides with the selective light reflection peak of the cholesteric mixture.

Using the spectral data and eqn (2), the dissymmetry factor, g_e, characterizing the degree of circular polarization of the emitted light was calculated.²³

g_e = 2(I_L − I_R)/(I_L + I_R) (2)

Fig. 4b shows the dependence of g_e on the wavelength of the cholesteric LC-gel before and after UV-irradiation. The negative sign of the dissymmetry factor in the range of selective light reflection corresponds to a lower intensity of the left-handed circularly polarized light that is explained by the “forbidden” penetration of the left-handed component through the cholesteric helical structure with the same handedness.^23–25

The photoinduced melting of the LC-gel improves the planar alignment in the LC-cell and simultaneously increases the absolute values of g_e (Fig. 4b). In other words, UV-irradiation and the photoinduced melting of the LC-gel are able to realize photomanipulation of the degree of circular polarization in such systems.

Conclusions

In conclusion, for the first time we have prepared photochromic LC-gels, based on a low-molar-mass bent-shaped gelator, which are capable of reversible melting under UV light action. So, the light irradiation allows one to manipulate phase behaviour, fluidity and optical properties of gels that can be used for the study of the unusual phase phenomena in LC systems. In addition, nematic and cholesteric mixtures with phototunable degrees of linearly or circularly-polarized fluorescence were elaborated, exploring the effect of the gel–sol transition studied in this work. Future studies will focus on the investigation of the peculiarities of the thermal and photo-optical behaviors of such systems, as well as on the search for new photochromic gelators for different LC matrices.

Experimental section

Synthesis and detailed characterization of new photochromic azobenzene-containing bent-shaped compound, 10BVIABr, are presented in the ESI (Fig. S1–S5†).

The polarizing optical microscopy investigations were performed using a LOMO P-112 polarizing microscope equipped with a Mettler TA-400 heating stage. DSC curves were obtained using a Perkin Elmer DSC 8500 calorimeter. Photochemical investigations were performed using an optical set up equipped with a DRSh-350 ultra-high pressure mercury lamp and a UV LED (380 nm). To prevent heating of the samples due to the IR irradiation of the mercury lamp, a water filter was introduced in the optical set-up. To assure a plane-parallel light beam, a quartz lens was applied. Using the filters, light with the wavelength of 436 nm was selected. The intensity of light was measured using a LaserMate-Q (Coherent) intensity meter and was equal to ∼8.0 mW cm⁻² (380 nm) for the LED and ∼1.0 mW cm⁻² (436 nm) for the lamp. Spectral measurements were performed using a Unicam UV-500 UV-Vis spectrophotometer.

Linearly and circularly polarized fluorescence was measured using an M266 spectrometer (Solar Laser Systems). A 532 nm diode laser, MGL-FN-532-1W, was used as the excitation source.

Acknowledgements

This research was supported by the Russian Science Foundation 14-13-00379 (photo-optical properties of the gels) and Czech Science Foundation CSF 13-14133S (synthesis of the bent-shaped compound). Authors are thankful to Mr Alexey Piryazev from the Moscow State University for the DSC measurements of the gels and Dr Damian Pociecha from the Warsaw University for performing the X-ray measurements of the bent-shaped LC compound.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07234d

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